The present invention relates to an apparatus and method for stabilizing a drifting or oversteered vehicle. Specifically, the present invention is directed to an improved braking system which results in better steerability and driving stability when braking in a curve.
Several methods and devices are known in the art for recognizing when and in which direction a vehicle is cornering. Furthermore, it is known to control braking of wheels individually based on wheel slip and wheel dynamics. For example, presently so-called select low principle anti-lock braking (xe2x80x9cABSxe2x80x9d) systems control front axle wheels of the vehicle individually and the rear axle wheels either individually or jointly. These known systems have several disadvantages in certain braking situations when the vehicle is turning.
For example, if a vehicle drifts during a turn, (i.e. the entire vehicle slides in the outward direction when the brakes are applied) usually all wheels are slipping. When drifting occurs during a braking action in a curve, at least the rear wheels and inside front wheel are likely to lock-up. The outer front wheel tends to be the least likely to lock up because the driving dynamics and weight distribution of most vehicles. In a vehicle having a known select low principle ABS system, the ABS controller responds to a drifting situation by completely removing the braking pressure from the locked wheels. When a high vehicle transverse acceleration causes maximum lateral forces at a wheel, that wheel can transmit little or no braking force to the vehicle without locking up. Thus, in most cases the braking pressure would be removed from both rear axle wheels, according to the select low control system, and the inside front wheel. In addition, the outside front wheel will have very little or no braking pressure, depending on whether the wheel is inside or outside the stability limit range. The stability limit range is a range in which the wheel has a tendency to lock. In other words, a range in which the wheel, but for a reduction in braking pressure, would lock up. To the driver in this situation, the brake pedal feels hard, yet the vehicle is not decelerating as quickly as it should.
In a vehicle that also has a chassis-related tendency to oversteer (i.e. the weight distribution is such that the rear of the vehicle tends to slide outwards during rapid cornering and braking), this tendency is reinforced even more by the lateral force provided at the front wheel on the outside of the turn.
According the physical interrelation of Kamm""s circle, the less braking force a wheel must transfer, the more lateral guiding force it is able to transfer. Consequently, the vehicle follows its steering input and has a tendency to oversteer. In known systems, the ABS system reduces the braking force at the outside wheel because of the existing slip angle and the resulting tire slip. In this driving situation, however, such a reduction of braking force causes the vehicle to further destabilize. A stable vehicle performance is only achieved in this situation in response to slightly overbraking the outside front wheel.
Known vehicle dynamic control systems use expensive additional sensory technology for detecting oversteering/understeering. The additional sensory technology used in these systems generally includes an admission pressure sensor, a yaw sensor, a lateral acceleration sensor, a longitudinal-acceleration sensor, and a steering-angle sensorxe2x80x94all devices not generally found in a typical ABS or TCS system (antilock braking system/traction control system). Such vehicle dynamic control systems are not used in conjunction with select-low controlled ABS systems because they allow more precise determination of the physical limits. A typical ABS/TCS which has only the wheel speeds available and using the select low principle, will control the rear axle during an unstable braking condition by controlling the more unstable wheel.
The present invention relates to a method and apparatus for providing a stability measure to counteract oversteering when a vehicle is braking in a turn at a limit cornering speed or to counteract drifting when a vehicle is braking in a turn. Specifically, the invention relates to a method or apparatus for controlling a braking system in a vehicle, the vehicle including an inside front wheel and outside front wheel, wherein a determination is made of whether a drifting and/or an oversteering condition exists; and, if so, a predetermined braking pressure is applied to the outside front wheel. The turning and/or braking condition of the vehicle can be evaluated to determine whether the drifting and/or oversteering condition exists, and such a condition is determined to exist when a transverse acceleration of the vehicle exceeds a predetermined transverse acceleration value or when a difference between a velocity of the inside front wheel a velocity of the outside front wheel exceeds a first predetermined velocity difference. Also, the drifting and/or oversteering condition is determined to exist if a difference between a velocity of an inside rear wheel and a velocity of an outside rear wheel exceeds a second predetermined velocity difference or when a velocity of the outside rear wheel exceeds a velocity of each of the remaining wheels by a predetermined amount. The predetermined braking pressure is greater than a respective braking pressure on each of the remaining wheels of the vehicle and may correspond to a driver-specifiable braking pressure.